A wide variety of signals and related protocols exist for the use of radio frequency (RF) signals in communication systems and other devices, such as radar systems. Prior receiver architectures for such RF communication systems are described in U.S. Pat. No. 7,436,910, entitled “DIRECT BANDPASS SAMPLING RECEIVERS WITH ANALOG INTERPOLATION FILTERS AND RELATED METHODS,” and U.S. patent application Ser. No. 7,436,912, entitled “NYQUIST FOLDED BANDPASS SAMPLING RECEIVERS AND RELATED METHODS,” each of which is hereby incorporated by reference in its entirety.
FIG. 1 (Prior Art) shows an embodiment for a reconfigurable direct RF bandpass sampling receiver (RDRFBSR), such as those described in U.S. Pat. No. 7,436,910. As depicted, the radio frequency (RF) input signal 116 is first passed through a low noise amplifier (LNA) 102. The output 130 of the LNA 102 is provided to a tunable/switchable bandpass filter 104, which can be configured to have a tunable center frequency and a programmable bandwidth dependent upon one or more filter control signals 105. The filtered output signal 132 is received by the non-quantizing sampler 204, which samples the signals at times determined by the RF sample clock 212 resulting in a discrete time continuous voltage sampled signal. The sampled signal is then filtered by the interpolation filter, resulting in a continuous time continuous voltage signal, which is then sampled and quantized by the ADC 210 at sample rate determined by the ADC Sample Clock 214 after optional amplification by the IF amp 208. The digital output signals from the ADC 210 are then further processed by digital signal processing (DSP) circuitry 114 to produce baseband in-phase path (I) and quadrature path (Q) signals. One limitation of this architecture, however, is that for very high RF input signals and operational conditions, the RF Sample Clock 212 jitter is amplified and may result in subsequent signal distortion in the form of signal spreading in the output signals, leading to significantly reduced SNR.
One limitation of this architecture of FIG. 1, however, is that for very high RF input signals and wide RF range operational conditions, developing a tunable/switchable filter over a wide RF range with high Q is very difficult and costly; thus it is difficult to control the filter skirts sufficiently to avoid signal leakage from adjacent Nyquist zones through the anti-alias filter. In addition, no provision is made for signal distortion in the form of ADC (analog-to-digital converter) induced spurs in the output signals.
FIG. 2 (Prior Art) shows an embodiment of a Nyquist folding receiver (NYFR) 200, such as those described in U.S. Pat. No. 7,436,912. The NYFR is similar to the RDRFBSR. Starting with the RDRFBSR, the anti-alias filter is replaced with a wideband pre-select filter, and the constant RF sample clock is replaced with a frequency modulated sample clock that samples at the zero-crossing rising voltage of a frequency modulated clock. In FIG. 2, an ultra wideband (UWB) front end filter 302 is present in front of a non-quantizing RF sampler 204 to allow reception of multiple Nyquist zones. The non-quantizing RF sampler 204 uses modulated RF sample clock circuitry 304, and is followed by an analog interpolation filter 206 and an analog to digital converter (ADC) 210. The ADC 210 receives an ADC sampling clock signal 214 from ADC clock circuitry. The wideband filter 302 has a bandwidth that is wide enough to pass multiple Nyquist zones where the Nyquist zones are determined by the RF sampling clock frequency for the non-quantizing RF sampler 204. The modulated sample clock circuitry 304 provides an RF sampling clock signal to the non-quantizing RF sampler 204 that is not constant and is adjusted or modulated during sampling.
As with RDRFBSR architectures, the NYFR architecture can suffer from ADC induced spurs as well as signal leakage outside the desired wideband pre-select filter bandwidth.
FIG. 3 (Prior Art) shows the input/output characteristics of the NYFR. In particular, an input signal has an induced modulation MΘ(t), where M depends on the Nyquist zone in which the signal originated. Thus, a broadband RF input can be sampled at far less than Nyquist, allowing individual signals from different Nyquist zones to alias (or fold) into the analog interpolation filter. The original RF frequency from which each signal aliased can then be determined without ambiguity by measuring M.
FIG. 4 (Prior Art) illustrates the principles of the NYFR via a frequency domain example. The Fourier transform of the pulse train, shown in the right side of FIG. 4 (Prior Art), is convolved with the input spectra after the wideband RF filter, which is shown at top left. It is noted that the Fourier transform of the pulse train consists of a series of impulse-like signals with increasing width. For example, the width at 0 fS1 is 0; the width at 1fS1 is the modulation bandwidth; the width at 2fS1 is 2×the modulation bandwidth; etc. When these are convolved with the input spectra, the resulting spectra has modulation bandwidth corresponding to Nyquist zone of origin as shown in the lower left hand side of FIG. 4 (Prior Art). It is noted that the numbers on the left side of FIG. 4 (Prior Art) correlate to the numbers in FIG. 2 (Prior Art) and show the positions within the circuitry where the signals in FIG. 4 (Prior Art) are present.
One problem suffered by the RDRFBSR and NYFR architectures can be ADC induced spurs. For reducing ADC spurs, prior attempts have focused on the ADC itself by attempting to provide linear and/or non-linear equalization thereby leading to ADCs with greater linearity. However, these ADCs suffer from higher cost and/or higher power requirements. Other prior attempts have been based on various dithering techniques, including ADC clock dithering, injection of low level white noise, and injection of out-of-band colored noise which is later removed by filtering. While these techniques help remove quantization spurs and help reduce some other non-linearities caused by sampling a pure periodic signal, they do not reduce ADC spurs adequately in most cases where high linear dynamic range is desired.